MESA: Microarchitecture Extensions for Spatial Architecture Generation

Modern heterogeneous CPUs incorporate hardware accelerators to enable domain-specialized execution and achieve improved efficiency. A well-known class among them, spatial accelerators, are designed with reconfigurability to accelerate a wide range of computeheavy and data-parallel applications. Unlike CPU cores, however, they tend to require specialized compilers and software stacks, libraries, or languages to operate and cannot be utilized with ease by all applications. As a result, the accelerator's large pool of compute and memory resources sit wastefully idle when it is not explicitly programmed. Our goal is to dismantle this CPU-accelerator barrier by monitoring CPU threads for acceleration opportunities during execution and, if viable, dynamically reconfigure the accelerator to allow transparent offloading. We develop MESA (Microarchitecture Extensions for Spatial Architecture Generation), a hardware block on the CPU that translates machine code to build an accelerator configuration specialized for the running program. While such a dynamic translation/reconfiguration approach is challenging, it has a key advantage over ahead-of-time compilers: access to runtime information, revealing not only dynamic dependencies but also performance characteristics. MESA maintains a real-time performance model of the program mapped on the accelerator in the form of a spatial dataflow graph with nodes weighted by operation latency and edges weighted by data transfer latency. Features of this dataflow graph are continuously updated with runtime information captured by performance counters, allowing a feedback loop of optimization, reconfiguration, and acceleration. This performance model allows MESA to identify the accelerator's critical paths and pinpoint its bottlenecks, upon which we implement in hardware a data-driven instruction mapping algorithm that locally minimizes latency. Backed by a synthesized RTL implementation, we evaluate the feasibility of our microarchitectural solution with different accelerator configurations. Across the Rodinia benchmarks, results demonstrate an average 1.3x speedup in performance and 1.8x gain in energy efficiency against a multicore CPU baseline.